A speech signal transmitter circuit

Abstract

The utility model discloses a voice signal transmitter circuit, including low frequency amplifier circuit, frequency synthesis oscillation circuit, resonant hybrid circuit and impedance matching transmitting circuit of connection in proper order, low frequency amplifier circuit receives voice input signal, impedance matching transmitting circuit output satisfies the high frequency small signal of transmitter internal antenna radiation frequency, the utility model discloses an advantage at: avoid signal interference in the voice transmission process, guarantee signal not distortion in the voice transmission process, improve the call quality.

Description

Technical Field

[0001] This utility model relates to the field of wireless transmission communication, and more specifically to a voice signal transmitter circuit. Background Technology

[0002] A transmitter is a device that transmits signals at a specific frequency; it's a broad concept widely used in various civilian and military equipment, including television, radio, communications, alarms, radar, remote control, telemetry, and electronic warfare. Transmitters can be categorized into four main types based on their modulation method: frequency modulation (FM), amplitude modulation (AM), phase modulation (PM), and pulse modulation. These can also be analog or digital. Typically, a transmitter consists of three parts: a high-frequency section, a low-frequency section, and a power supply section. The high-frequency section generally includes a main oscillator, a buffer amplifier, a frequency multiplier, intermediate amplifiers, a power amplifier driver stage, and a final power amplifier stage. The main oscillator generates a stable carrier wave. To improve frequency stability, a quartz crystal oscillator is often used in the main oscillator stage, followed by a buffer stage to reduce the impact of subsequent stages on the main oscillator. The low-frequency section includes a microphone, a low-frequency voltage amplifier stage, a low-frequency power amplifier stage, and a final low-frequency power amplifier stage. The low-frequency signal is gradually amplified to obtain the required power level at the final power amplifier stage, enabling modulation of the high-frequency final power amplifier stage. Therefore, the final low-frequency power amplifier stage is also called a modulator. Modulation is the process of loading the information to be transmitted onto a high-frequency oscillation (carrier frequency) signal. Therefore, the final high-frequency power amplifier stage is called a modulated amplifier.

[0003] In wireless voice transmission transceiver systems, the transmitter plays a crucial role. The signal emitted by the transmitter is directly received, processed, and output by the receiver. Therefore, the stability of the transmitter's signal directly affects the voice transmission quality of the entire system. In existing wireless voice transmission transceiver systems, the transmitter's anti-interference processing of the received voice signal is not very effective, resulting in significant interference in the output signal. This directly affects the receiver's performance, leading to signal distortion and decreased call quality during voice transmission. Utility Model Content

[0004] The technical problem to be solved by this invention is that the existing transmitter has significant signal interference, which directly affects the receiver's performance, resulting in signal distortion and reduced call quality during voice transmission.

[0005] This utility model solves the above-mentioned technical problems through the following technical means: a voice signal transmitter circuit, including a low-frequency amplifier circuit, a frequency synthesis oscillation circuit, a resonant hybrid circuit, and an impedance matching transmitter circuit connected in sequence. The low-frequency amplifier circuit includes a filter unit for filtering out signal interference, and the impedance matching transmitter circuit includes a π-type filter for further filtering out signal interference. The low-frequency amplifier circuit receives the voice input signal, and the impedance matching transmitter circuit outputs a high-frequency small signal that satisfies the radiation frequency of the transmitter's internal antenna.

[0006] This invention features a low-frequency amplification circuit at the signal input end, which includes a filtering unit for filtering out signal interference. An impedance matching transmission circuit is also provided at the signal output end, which includes a π-type filter for further filtering out signal interference. Multiple filtering processes are performed throughout the signal transmission process to ensure the stability of signal transmission, avoid interference from other signals besides voice signals, ensure that the signal is not distorted during voice transmission, and improve call quality.

[0007] Furthermore, the low-frequency amplifier circuit also includes a signal input unit, a first-stage amplifier unit, and a second-stage amplifier unit, which are connected together. The filter unit is connected to both the first-stage amplifier unit and the second-stage amplifier unit.

[0008] Furthermore, the signal input unit is a microphone MK1, the first-stage amplification unit is a transistor VT4, and the second-stage amplification unit is a transistor VT5. The microphone MK1 receives two amplitude-modulated dual-channel voice input signals. One end of the microphone MK1 is connected to the base of the transistor VT4, the collector of the transistor VT4 is connected to the base of the transistor VT5, the collector of the transistor VT5 is connected to the frequency synthesis oscillation circuit, and the emitters of the transistor VT4 and VT5 are connected to the other end of the microphone MK1.

[0009] Furthermore, the filtering unit includes capacitors C1, C2, and C3, resistors R1 to R4, R9, R10, and R12 numbered sequentially, capacitors C13, C14, and C6. One end of the microphone MK1 is connected to one end of capacitor C13 and one end of resistor R1. The other end of capacitor C13 is connected to one end of resistor R9 and the base of transistor VT4. The other end of resistor R9, one end of resistor R2, the collector of transistor VT4, and one end of capacitor C14 are connected. The other end of capacitor C14 is connected to one end of resistor R10 and the base of transistor VT5. Connect the other end of resistor R10, one end of resistor R3, the positive terminal of capacitor C6, and the collector of transistor VT5. Connect the negative terminal of capacitor C6 to the frequency synthesis oscillation circuit. Connect one end of resistor R12, the emitter of transistor VT5, the emitter of transistor VT4, and the other end of microphone MK1 to ground. Connect the other end of resistor R12 to one end of resistor R4. Connect the other ends of resistors R1 to R4, the positive terminals of capacitors C1 and C2, and one end of capacitor C3. Connect one end of capacitor C3 to the resonant hybrid circuit. Connect the negative terminals of capacitors C1 and C2, and the other end of capacitor C3 to ground.

[0010] Furthermore, the frequency synthesis oscillation circuit includes a resistor R11, a crystal oscillator JT1, and a diode VD1. One end of the resistor R11 is connected to the negative terminal of the capacitor C6, and the other end of the resistor R11 is connected to one end of the crystal oscillator JT1 and the cathode of the diode VD1, respectively. The other end of the crystal oscillator JT1 and the anode of the diode VD1 are both connected to the resonant hybrid circuit.

[0011] Furthermore, the resonant hybrid circuit includes a frequency selection unit, a Class C amplifier, and a buffer amplifier, which are connected sequentially.

[0012] Furthermore, the frequency selection unit includes resistor R6, resistor R14, capacitor C15, transistor VT1, capacitor C4, resistor R15, capacitor C20, inductor L1, and capacitor C12. One end of resistor R6, one end of resistor R14, one end of capacitor C15, and the base of transistor VT1 are connected to the other end of crystal oscillator JT1. The collector of transistor VT1, one end of capacitor C4, one end of inductor L1, and one end of capacitor C12 are connected. The emitter of transistor VT1, the other end of capacitor C15, one end of resistor R15, and one end of capacitor C20 are connected. The other end of resistor R6, the other end of capacitor C4, and the other end of inductor L1 are connected to one end of capacitor C3. The other ends of resistor R14, resistor R15, and capacitor C20 are all connected to the anode of diode VD1.

[0013] Furthermore, the Class C amplifier includes resistor R7, resistor R16, capacitor C21, transistor VT2, capacitor C5, inductor L2, and capacitor C7. The other end of capacitor C12, one end of resistor R7, one end of resistor R16, one end of capacitor C21, and the base of transistor VT2 are connected. The collector of transistor VT2, one end of capacitor C5, one end of inductor L2, and one end of capacitor C7 are connected. The other ends of resistor R7, capacitor C5, and inductor L2 are all connected to the other end of inductor L1. The other ends of resistor R16, capacitor C21, and the emitter of transistor VT1 are all connected to the other end of capacitor C20.

[0014] Furthermore, the buffer amplifier includes a resistor R17, a capacitor C22, a transistor VT3, and an inductor L3. The other end of the capacitor C7, one end of the resistor R17, one end of the capacitor C22, and the base of the transistor VT3 are connected. The collector of the transistor VT3 is connected to one end of the inductor L3. The other end of the inductor L3 is connected to the other end of the inductor L2. The other end of the resistor R17, the other end of the capacitor C22, and the emitter of the transistor VT3 are all connected to the emitter of the transistor VT2. The other end of the inductor L3 and the collector of the transistor VT3 are both connected to the impedance matching emitter circuit.

[0015] Furthermore, the impedance matching transmitting circuit also includes a resistor R8, an indicator LED1, capacitors C10 and C11, and an antenna E1. The π-type filter includes capacitors C8, C9, C16, C17, C18, C19, inductors L4, L5, and L6. One end of capacitor C8 is connected to one end of inductor L3, and the other end of capacitor C8, one end of inductor L4, and one end of capacitor C16 are connected together. The other end of inductor L4 is connected to one end of capacitor C9, and the other end of capacitor C9, one end of inductor L5, and one end of capacitor C16 are connected together. One end of 17 is connected to the other end of inductor L5, one end of inductor L6, and one end of capacitor C18. The other end of inductor L6, antenna E1, and one end of capacitor C19 are connected. One end of resistor R8, the positive terminal of capacitor C10, and one end of capacitor C11 are all connected to the other end of inductor L3 and connected to the +6V power supply. The other end of resistor R8 is connected to the anode of indicator LED1. The emitter of transistor VT3, the other ends of capacitors C16 to C19, the cathode of indicator LED1, the negative terminal of capacitor C10, and the other end of capacitor C11 are all connected and grounded.

[0016] The advantages of this utility model are:

[0017] (1) This utility model sets a low-frequency amplification circuit at the signal input end, which includes a filter unit for filtering out signal interference. An impedance matching transmission circuit is set at the signal output end, which includes a π-type filter for further filtering out signal interference. Multiple filtering processes are performed throughout the signal transmission process to ensure the stability of signal transmission, avoid interference from other signals besides voice signals, ensure that the signal is not distorted during voice transmission, and improve call quality.

[0018] (2) In the low-frequency amplifier circuit of this utility model, the microphone MK1 receives two dual-channel voice input signals after amplitude modulation. The frequency synthesis oscillation circuit modulates two different frequencies and loads them onto the dual-channel voice input signals respectively, thereby increasing the frequency of the voice signal, facilitating signal transmission, and achieving high spectrum utilization and high communication efficiency.

[0019] (3) The low-frequency amplifier circuit of this utility model amplifies the weak voice signal captured by the microphone and inputs it to the next stage, ensuring that the amplified signal is not distorted during the amplification process.

[0020] (4) In the resonant hybrid circuit of this utility model, transistor VT1 and capacitor C4 select the third frequency, transistor VT2 and capacitor C5 form a class C amplifier to amplify the signal, and transistor VT3 and inductor L3 form a buffer amplifier circuit to buffer and amplify the signal.

[0021] (5) The impedance matching transmitting circuit of this utility model filters out small signal interference through step-by-step filtering. The Π-type impedance matching network minimizes the input impedance so that the antenna can obtain the maximum output power and greatly reduce the antenna radiation loss. Attached Figure Description

[0022] Figure 1 This is a structural block diagram of a voice signal transmitter circuit disclosed in an embodiment of the present utility model;

[0023] Figure 2 This is a schematic diagram of a low-frequency amplifier circuit in a voice signal transmitter circuit disclosed in an embodiment of the present utility model;

[0024] Figure 3 This is a schematic diagram of a frequency synthesis oscillation circuit in a voice signal transmitter circuit disclosed in an embodiment of the present utility model;

[0025] Figure 4 This is a schematic diagram of a resonant hybrid circuit in a voice signal transmitter circuit disclosed in an embodiment of the present utility model;

[0026] Figure 5 This is a schematic diagram of an impedance matching transmitting circuit in a voice signal transmitter circuit disclosed in an embodiment of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this utility model clearer, the technical solutions of the embodiments of this utility model will be clearly and completely described below in conjunction with the embodiments of this utility model. Obviously, the described embodiments are only some embodiments of this utility model, not all embodiments. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this utility model.

[0028] like Figure 1 As shown, a voice signal transmitter circuit includes a low-frequency amplifier circuit 1, a frequency synthesis oscillation circuit 2, a resonant hybrid circuit 3, and an impedance matching transmitter circuit 4 connected in sequence. The low-frequency amplifier circuit 1 receives the voice input signal, and the impedance matching transmitter circuit 4 outputs a high-frequency small signal that satisfies the radiation frequency of the transmitter's internal antenna.

[0029] like Figure 2 As shown, the low-frequency amplifier circuit 1 includes capacitors C1, C2, and C3; resistors R1 to R4 numbered sequentially; microphone MK1; resistors R9, R10, and R12; capacitors C13, C14, and C6; transistor VT4; and transistor VT5. Microphone MK1 receives two amplitude-modulated dual-channel voice input signals. The front end of microphone MK1 can be connected to a 30kHz amplitude modulation circuit and a 40kHz amplitude modulation circuit. The amplitude-modulated dual-channel voice signals are then transmitted to the low-frequency amplifier circuit 1 through microphone MK1. One end of microphone MK1 is connected to one end of capacitor C13 and one end of resistor R1. The other end of capacitor C13 is connected to one end of resistor R9 and the base of transistor VT4. The other end of resistor R9 and one end of resistor R2... The collector of transistor VT4 is connected to one end of capacitor C14. The other end of capacitor C14 is connected to one end of resistor R10 and the base of transistor VT5. The other end of resistor R10, one end of resistor R3, the positive terminal of capacitor C6, and the collector of transistor VT5 are connected. The negative terminal of capacitor C6 is connected to frequency synthesis oscillation circuit 2. One end of resistor R12, the emitter of transistor VT5, the emitter of transistor VT4, and the other end of microphone MK1 are connected and grounded. The other end of resistor R12 is connected to one end of resistor R4. Resistors R1 to the other end of resistor R4, the positive terminals of capacitors C1 and C2, and one end of capacitor C3 are all connected. One end of capacitor C3 is connected to resonant mixing circuit 3. The negative terminals of capacitors C1 and C2, and the other end of capacitor C3 are grounded. Transistors VT4 and VT5 amplify the input voice signal in two stages. Resistors and capacitors form a filtering circuit to ensure the stability of signal transmission and filter out signal interference.

[0030] like Figure 3 As shown, the frequency synthesis oscillation circuit 2 includes a resistor R11, a crystal oscillator JT1, and a diode VD1. One end of the resistor R11 is connected to the negative terminal of the capacitor C6, and the other end of the resistor R11 is connected to one end of the crystal oscillator JT1 and the cathode of the diode VD1. The other end of the crystal oscillator JT1 and the anode of the diode VD1 are both connected to the resonant hybrid circuit 3. The frequency of the crystal oscillator JT1 is one-third of 48.5MHz. The frequency synthesis oscillation circuit 2 modulates two different frequencies and applies them to the dual-channel voice input signals to increase the frequency of the voice signals and facilitate signal transmission.

[0031] like Figure 4As shown, the resonant hybrid circuit 3 includes resistor R6, capacitor C4, inductor L1, resistor R7, capacitor C5, inductor L2, capacitor C7, inductor L3, transistors VT1 to VT3 numbered sequentially, capacitor C12, capacitor C15, resistors R14 to R17 numbered sequentially, capacitor C20, capacitor C21, and capacitor C22. One end of resistor R6, one end of resistor R14, one end of capacitor C15, and the base of transistor VT1 are connected and connected to the other end of crystal oscillator JT1. The collector of transistor VT1, one end of capacitor C4, one end of inductor L1, and one end of capacitor C12 are connected. The emitter of transistor VT1, the other end of capacitor C15, one end of resistor R15, and one end of capacitor C20 are connected. The other end of capacitor C12, one end of resistor R7, one end of resistor R16, and capacitor C22 are connected. One end of 21 is connected to the base of transistor VT2; the collector of transistor VT2, one end of capacitor C5, one end of inductor L2, and one end of capacitor C7 are connected; the other end of capacitor C7, one end of resistor R17, one end of capacitor C22, and the base of transistor VT3 are connected; the collector of transistor VT3 is connected to one end of inductor L3; the other ends of resistor R14, resistor R17, capacitor C20, capacitor C21, capacitor C22, and the emitter of transistor VT3 are all connected together; the other ends of resistor R6, capacitor C4, inductor L1, resistor R7, capacitor C5, inductor L2, and inductor L3 are all connected together; the other end of inductor L3 and the emitter of transistor VT3 are connected to impedance matching emitter circuit 4. Transistor VT1 and capacitor C4 form a frequency multiplier circuit to select the third frequency of the resonant hybrid circuit 3. Transistor VT2 and capacitor C5 form a class C amplifier for power amplification. Transistor VT3 and inductor L3 form a buffer amplifier circuit to further buffer and amplify the signal. In this way, the signal after step-by-step amplification can meet the radiation frequency of the antenna, so that the antenna E2 of the receiving part can receive the signal transmitted by the transmitting part. Without signal amplification, the signal is too weak. In addition, there is a certain distance between the transmitting part and the receiving part during transmission, which can easily make it difficult for the receiving part to receive the voice signal.

[0032] like Figure 5As shown, the impedance matching transmitting circuit 4 includes capacitors C8 to C11 (numbered sequentially), inductors L4, L5, and L6, resistor R8, capacitors C16 to C19 (numbered sequentially), indicator LED1, and antenna E1. One end of capacitor C8 is connected to one end of inductor L3, and the other end of capacitor C8, one end of inductor L4, and one end of capacitor C16 are connected. The other end of inductor L4 is connected to one end of capacitor C9, and the other end of capacitor C9, one end of inductor L5, and one end of capacitor C17 are connected. Inductor L5... The other end of the inductor is connected to one end of the inductor L6 and one end of the capacitor C18. The other end of the inductor L6 is connected to the antenna E1 and one end of the capacitor C19. One end of the resistor R8, the positive terminal of the capacitor C10, and one end of the capacitor C11 are all connected to the other end of the inductor L3 and connected to the +6V power supply. The other end of the resistor R8 is connected to the anode of the indicator LED1. The emitter of the transistor VT3, the other ends of capacitors C16 to C19, the cathode of the indicator LED1, the negative terminal of capacitor C10, and the other end of capacitor C11 are all connected to and grounded. The impedance matching transmitter circuit 4 filters out small signal interference through step-by-step filtering. The Π-type impedance matching network minimizes the input impedance, so that the antenna can obtain the maximum output power and greatly reduce the antenna radiation loss.

[0033] Through the above technical solution, this utility model sets a low-frequency amplifier circuit 1 at the signal input end, which includes a filter unit for filtering out signal interference. An impedance matching transmission circuit 4 is set at the signal output end, which includes a π-type filter for further filtering out signal interference. Multiple filtering processes are performed throughout the signal transmission process to ensure the stability of signal transmission, avoid interference from other signals besides voice signals, ensure that the signal is not distorted during voice transmission, and improve call quality.

[0034] The above embodiments are only used to illustrate the technical solutions of this utility model, and are not intended to limit it. Although this utility model has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this utility model.

Claims

1. A voice signal transmitter circuit, characterized in that, The device includes a low-frequency amplifier circuit, a frequency synthesis oscillation circuit, a resonant hybrid circuit, and an impedance matching transmitter circuit connected in sequence. The low-frequency amplifier circuit includes a filter unit for filtering out signal interference, and the impedance matching transmitter circuit includes a π-type filter for further filtering out signal interference. The low-frequency amplifier circuit receives a voice input signal, and the impedance matching transmitter circuit outputs a high-frequency small signal that satisfies the radiation frequency of the transmitter's internal antenna.

2. The voice signal transmitter circuit according to claim 1, characterized in that, The low-frequency amplifier circuit further includes a signal input unit, a first-stage amplifier unit, and a second-stage amplifier unit, which are connected together. The filter unit is connected to both the first-stage amplifier unit and the second-stage amplifier unit.

3. The voice signal transmitter circuit according to claim 2, characterized in that, The signal input unit is a microphone MK1, the first-stage amplification unit is a transistor VT4, and the second-stage amplification unit is a transistor VT5. The microphone MK1 receives two amplitude-modulated dual-channel voice input signals. One end of the microphone MK1 is connected to the base of transistor VT4, the collector of transistor VT4 is connected to the base of transistor VT5, the collector of transistor VT5 is connected to a frequency synthesis oscillation circuit, and the emitters of transistor VT4 and VT5 are connected to the other end of the microphone MK1.

4. The voice signal transmitter circuit according to claim 3, characterized in that, The filtering unit includes capacitors C1, C2, and C3, and resistors R1 to R4, R9, R10, and R12 numbered sequentially, as well as capacitors C13, C14, and C6. One end of the microphone MK1 is connected to one end of capacitor C13 and one end of resistor R1. The other end of capacitor C13 is connected to one end of resistor R9 and the base of transistor VT4. The other end of resistor R9, one end of resistor R2, the collector of transistor VT4, and one end of capacitor C14 are connected. The other end of capacitor C14 is connected to one end of resistor R10 and the base of transistor VT5. The other end of resistor R10, one end of resistor R3, the positive terminal of capacitor C6, and the collector of transistor VT5 are connected. The negative terminal of capacitor C6 is connected to the frequency synthesis oscillation circuit. One end of resistor R12, the emitter of transistor VT5, the emitter of transistor VT4, and the other end of microphone MK1 are connected and grounded. The other end of resistor R12 is connected to one end of resistor R4. The other ends of resistors R1 to R4, the positive terminals of capacitors C1 and C2, and one end of capacitor C3 are all connected. One end of capacitor C3 is connected to the resonant hybrid circuit. The negative terminals of capacitors C1 and C2, and the other end of capacitor C3 are grounded.

5. A voice signal transmitter circuit according to claim 4, characterized in that, The frequency synthesis oscillation circuit includes a resistor R11, a crystal oscillator JT1, and a diode VD1. One end of the resistor R11 is connected to the negative terminal of the capacitor C6, and the other end of the resistor R11 is connected to one end of the crystal oscillator JT1 and the cathode of the diode VD1. The other end of the crystal oscillator JT1 and the anode of the diode VD1 are both connected to the resonant hybrid circuit.

6. The voice signal transmitter circuit according to claim 5, characterized in that, The resonant hybrid circuit includes a frequency selection unit, a Class C amplifier, and a buffer amplifier, which are connected in sequence.

7. A voice signal transmitter circuit according to claim 6, characterized in that, The frequency selection unit includes resistor R6, resistor R14, capacitor C15, transistor VT1, capacitor C4, resistor R15, capacitor C20, inductor L1, and capacitor C12. One end of resistor R6, one end of resistor R14, one end of capacitor C15, and the base of transistor VT1 are connected to the other end of crystal oscillator JT1. The collector of transistor VT1, one end of capacitor C4, one end of inductor L1, and one end of capacitor C12 are connected. The emitter of transistor VT1, the other end of capacitor C15, one end of resistor R15, and one end of capacitor C20 are connected. The other ends of resistor R6, capacitor C4, and inductor L1 are connected to one end of capacitor C3. The other ends of resistor R14, resistor R15, and capacitor C20 are all connected to the anode of diode VD1.

8. A voice signal transmitter circuit according to claim 7, characterized in that, The Class C amplifier includes resistors R7 and R16, capacitor C21, transistor VT2, capacitor C5, inductor L2, and capacitor C7. The other end of capacitor C12, one end of resistor R7, one end of resistor R16, one end of capacitor C21, and the base of transistor VT2 are connected. The collector of transistor VT2, one end of capacitor C5, one end of inductor L2, and one end of capacitor C7 are connected. The other ends of resistor R7, capacitor C5, and inductor L2 are all connected to the other end of inductor L1. The other ends of resistor R16, capacitor C21, and the emitter of transistor VT1 are all connected to the other end of capacitor C20.

9. A voice signal transmitter circuit according to claim 8, characterized in that, The buffer amplifier includes a resistor R17, a capacitor C22, a transistor VT3, and an inductor L3. The other end of the capacitor C7, one end of the resistor R17, one end of the capacitor C22, and the base of the transistor VT3 are connected. The collector of the transistor VT3 is connected to one end of the inductor L3. The other end of the inductor L3 is connected to the other end of the inductor L2. The other end of the resistor R17, the other end of the capacitor C22, and the emitter of the transistor VT3 are all connected to the emitter of the transistor VT2. The other end of the inductor L3 and the collector of the transistor VT3 are both connected to the impedance matching emitter circuit.

10. A voice signal transmitter circuit according to claim 9, characterized in that, The impedance matching transmitting circuit further includes a resistor R8, an indicator LED1, capacitors C10 and C11, and an antenna E1. The π-type filter includes capacitors C8, C9, C16, C17, C18, C19, inductors L4, L5, and L6. One end of capacitor C8 is connected to one end of inductor L3, and the other end of capacitor C8, one end of inductor L4, and one end of capacitor C16 are connected together. The other end of inductor L4 is connected to one end of capacitor C9, and the other end of capacitor C9, one end of inductor L5, and one end of capacitor C17 are connected together. One end is connected to the other end of inductor L5, one end of inductor L6, and one end of capacitor C18. The other end of inductor L6, antenna E1, and one end of capacitor C19 are connected. One end of resistor R8, the positive terminal of capacitor C10, and one end of capacitor C11 are all connected to the other end of inductor L3 and connected to the +6V power supply. The other end of resistor R8 is connected to the anode of indicator LED1. The emitter of transistor VT3, the other ends of capacitors C16 to C19, the cathode of indicator LED1, the negative terminal of capacitor C10, and the other end of capacitor C11 are all connected and grounded.

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